Cerebral palsy (CP) is the most prevalent physical disability occurring in childhood, for which no cure is available. All current treatments such a surgery and physical therapy aim solely to alleviate the peripheral effects of this central nervous system disorder, often with limited and inconsistent success. A prevalent clinical system that has long been recognized, but rarely addressed therapeutically in this population is muscle weakness. While strength training is routinely used in the adult orthopaedic population and in athletes to increase force production or alter muscle imbalance, this type of intervention is grossly under- utilized in pediatric rehabilitation, and in particular children with CP. No evidence exists to support the clinical prejudice against strength training and testing in cerebral palsy. In fact, research findings are accumulating toe the contrary, demonstrating that individuals with CP are indeed weak, the degree of weakness has a direct relationship to motor performance, and strengthening programs can produce positive functional outcomes. However, the neurophysiological and biomechanical bases of weakness in CP remain poorly understood. An additional concern is that the two major surgical interventions in ambulatory children with spastic diplegia, muscle-tendon lengthening (orthopaedic surgery) and selective dorsal rhizotomy (neurosurgery), often serve to exacerbate or unmask the symptom of weakness, which can be problematic in children who are already weak. The ultimate goal of this project is to improve motor outcomes in CP by first exploring the mechanisms that lead to diminished force production and defining the relative role of weakness in the functional motor deficit in spastic CP. This will be accomplished by isokinetic and electromyographic evaluation to examine voluntary and involuntary muscle responses in children with CP at the knee and ankle joint throughout the range of motion and at different movement speeds. Muscle performance will then be related to validated pediatric measures of functional status and disability. Lastly, the interaction of strength with surgical interventions will be determined by quantifying the effects of orthopaedic and neurosurgery on strength and functional measures, and assessing whether the addition of a strengthening program enhances surgical outcomes. Strength is an essential component of normal motor control that is deficient in CP, but can be altered through training. Continued research, as proposed here, needs to be conducted to verify and solidify the role of strengthening in the rehabilitation of children with CP.